What is the origin of Mycobacterium avium complex (MAC) lung disease?

From the Guidelines

Mycobacterium avium complex (MAC) lung disease originates from environmental sources, with individuals typically developing the disease after inhaling MAC bacteria from their environment, which then colonize and multiply in the lungs. The most recent and highest quality study, published in 2020 1, supports the notion that MAC lung disease is caused by the inhalation of these bacteria from environmental sources. According to this study, MAC bacteria are found throughout the environment, and the most common clinical presentation is pulmonary disease, often occurring in individuals with underlying structural airway disease.

Key factors that increase the risk of developing MAC lung disease include:

• Pre-existing lung conditions, such as bronchiectasis, COPD, or prior tuberculosis, which provide favorable conditions for bacterial growth
• Compromised immune systems, particularly in individuals with HIV/AIDS
• Certain demographic groups, including slender postmenopausal women (Lady Windermere syndrome) and middle-aged or elderly men with chronic lung disease

The bacteria's waxy cell wall makes them resistant to common disinfectants and antibiotics, allowing them to persist in the environment and making infections difficult to treat once established. As noted in the 2007 study 1, the mechanism of acquisition of the organism is not known, although it is assumed to be through ingestion of the organism from an environmental source.

In terms of treatment, the 2020 study 1 highlights the importance of identifying NTM to the species level and, in the case of M. abscessus, to the subspecies level, as this can provide important clinical and epidemiologic information. Treatment of NTM pulmonary disease varies depending on the species, extent of disease, drug susceptibility results, and underlying co-morbidities, and often requires the use of multiple antimicrobial agents.

Overall, the evidence suggests that MAC lung disease is a complex condition that requires careful diagnosis and treatment, and that individuals with certain risk factors, such as pre-existing lung conditions or compromised immune systems, are at higher risk of developing the disease.

From the Research

Origin of Mycobacterium Avium Complex (MAC) Lung Disease

• The exact origin of Mycobacterium avium complex (MAC) lung disease is not fully understood, but it is known that MAC organisms are ubiquitous in the environment 2, 3.
• MAC lung disease is thought to occur when an individual is exposed to MAC organisms in the environment, and some degree of susceptibility due to underlying lung disease or immunosuppression is required for the disease to develop 3.
• The frequency of pulmonary MAC disease is increasing in many areas, and the exact reasons are unknown 3.
• Reinfection with a new genotype from the environment is a significant cause of recurrence of MAC-pulmonary disease (PD), with recurrence rates reported to be as high as 25-45% 2.

Risk Factors for MAC Lung Disease

• Underlying lung disease, such as chronic obstructive pulmonary disease (COPD) or bronchiectasis, is a risk factor for MAC lung disease 4, 5.
• Immunocompromised individuals, such as those with HIV/AIDS, are also at increased risk of developing MAC lung disease 3.
• Other risk factors include low body mass index, poor nutritional status, presence of cavitary lesion(s), and extensive disease 5.

Environmental Sources of MAC

• MAC organisms can be found in a variety of environmental sources, including water, soil, and air 3.
• The exact source of MAC infection is often unclear, but it is thought that exposure to contaminated water or soil may play a role in the development of MAC lung disease 2, 3.